1. Field of the Invention
The present invention relates to plastic composites made from at least two incompatible plastics, A and B, in a multi-layer composite, in which the layering sequence alternates between A and B, the layers of plastic B are discontinued at regular intervals, and the resulting gaps in the B layers are filled in with plastic A. The present invention also relates to methods and apparatus for preparing such multilayer plastic composites.
2. Discussion of the Background
Plastic composites are well known. The primary goal in their production is good adhesion between the composite elements. One of the technically important fields of this type of composites is the reinforcement of plastic sheets using fibers or bands.
For example, DE-OS 38 35 575 describes a method for producing continuous, molded elements from units made of reinforcement fibers that have been preimpregnated with thermoplastic plastics, in which the fibers are shaped by melting the thermoplastics. The preimpregnation results in a good adhesion of the reinforcement fibers in the thermoplastic matrix.
DE-OS 38 40 374 discloses thermoplastic fiber-reinforced composites, which can be obtained by preimpregnating reinforcement fibers with a molten mass of thermoplastics, polyamides, a low-molecular weight acid amide and, if necessary, a coupling agent, along with methods for producing these and possible applications. The composites are prepared by a process in which a mixture of thermoplastic, polyamide, acid amide, and, if necessary, a coupling agent in the form of a prefabricated film or a freshly extruded molten film is used and, together with the reinforcement fibers, which in this case are in the form of a mat or unidirectional filament strands, are fed into a continuous mold. In this case, as well, good adhesion between the fibers and the thermoplastic matrix is of great importance.
U.S. Pat. No. 4,058,581 describes a process for the continuous production of molded elements that are reinforced with graphite fibers, in which the individual graphite fibers are first drawn through a solution of a thermoplastic resin, preferably a polypropylene that is grafted with acrylic acid, to effect preimpregnation with a coupling agent.
In EP-A 0,282,199, a process for the production of fiber composites made of a thermoplastic matrix and unidirectional reinforcement fibers is described, in which the fibers are preimpregnated with molten thermoplastic. In this case, continuously parallel fiber bundles that are not connected mechanically and a thermoplastic plastic are fed into a twin mold, where they are subjected to great pressure at a high temperature for a specific amount of time, after which they are cooled. In this process the fibers are completely and simultaneously preimpregnated, which allows a high fiber content of greater than 50% by volume in the thermoplastic matrix. In this case as well, good adhesion between the fibers and the thermoplastic matrix is of primary importance.
EP-PS 0,407,852 describes sheets made of acrylic glass that are suitable for use as sound-proofing elements, and which have embedded in them, approximately in the center, monofilament synthetic fibers, or a lattice web made of such fibers, which are preferably polyamide fibers. The polyamide fibers show no appreciable loss in tear resistance, since the adhesion between these types of fibers and the acrylic glass that surrounds them is relatively light.
Up to EP-PS 0,407,852, the patent specifications and published applications referred to above describe exclusively plastic composites that include layers of coupling agents. The use of additional coupling agents generally necessitates great technical expenditure, for example on feed devices for the coupling agents or a special melting and/or extrusion guiding device in the case of coextruded plastic composites. In addition, the adhesion that can be obtained in the plastic composite over layers of coupling agent is limited.
Thus, there remains a need for a method for combining incompatible plastics, and more importantly their mechanical properties, without the use of coupling agents. The application referred to above, EP-PS 0,407,852, attains this object only to a limited extent and in a very complex manner, and the discontinuous production of the plastic composites made of acrylic glass and synthetic fibers is very costly.
Accordingly, it is one object of the present invention to provide novel multilayer plastic composites.
It is another object of the present invention to provide multilayer plastic composites which contain layers of at least two incompatible plastics.
It is another object of the present invention to provide multilayer plastic composites which contain layers of at least two incompatible plastics and which exhibit improved mechanical properties.
It is another object of the present invention to provide a novel method for preparing such multilayer plastic composites.
It is another object of the present invention to provide novel apparatus for preparing such multilayer plastic composites.
These and other objects, which will become apparent during the following detailed description, have been achieved by the inventors"" discovery that multilayer plastic composites made of at least two incompatible plastics, A and B, can be produced if the layering sequence alternates between A and B, and if the layer of plastic B is discontinued at regular intervals and the resulting gaps in the layer are filled in with plastic A. In a preferred embodiment of the invention, the layers of plastic B are inserted as unidirectional columns in the plastic A.
In a further preferred embodiment of the invention the plastic B has a greater thermal expansion than the plastic A, whereby the plastic A is most preferably amorphous and the plastic B is most preferably amorphous, crystalline, or semi-crystalline. The composites are preferably produced via coextrusion, in which the plastic B is most preferably processed in an oscillating stream, which results in periodic changes in the cross-sections of the layer segments made of plastic B.